A spark plug used for ignition of an internal combustion engine, such as an automobile engine, generally includes: a tubular metallic shell; a tubular insulator disposed in a bore of the metallic shell; a center electrode disposed in a bore of the insulator at the forward end of the bore; and a ground electrode with one end joined to the forward end of the metallic shell and the other end forming a spark discharge gap between the other end and the center electrode. With the spark plug, spark discharge occurs in the spark discharge gap formed between the distal end of the center electrode and the forward end of the ground electrode within a combustion chamber of an internal combustion engine to thereby ignite fuel injected into the combustion chamber.
For the purpose of improving the durability of spark plugs, noble metal tips formed from a noble metal alloy have conventionally been provided on the discharge surface of the ground electrode and the discharge surface of the center electrode that face each other. However, increasing the degree of compression within combustion chambers and lean burn have recently become mainstream practices, and the use environment of spark plugs has become increasingly severe. Therefore, there is demand for further development of spark plugs that can maintain durability even in such a severe environment.
Even when a tip formed of high-melting point Ir alloy is used as the material of a spark discharge electrode, the resistance of the tip to spark wear is insufficient. To address this problem, Japanese Patent Application Laid-Open (kokai) No. 2002-93547, for example, has as its object to improve the heat dissipation of an Ir alloy tip (see paragraphs 0003 to 5 in Japanese Patent Application Laid-Open (kokai) No. 2002-93547). The solution to the problem described in Japanese Patent Application Laid-Open (kokai) No. 2002-93547 is “the spark plug being characterized in that the Ir alloy tip is embedded in an end portion of the ground electrode with part of the Ir alloy tip exposing from the discharge surface and that, when the discharge surface is viewed from the top, a side edge (47) of the Ir alloy tip coincides with an outer circumferential edge of the discharge surface or is located inward of the outer circumferential edge of the discharge surface” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 2002-93547).
One possible method of ensuring sufficiently high wear resistance of a noble metal tip of a spark plug to extend its service life even in the recent severe use environment of the spark plug is to increase the diameter of the noble metal tip. However, when a noble metal tip with an increased diameter is laser-welded to an electrode in an ordinary manner, the noble metal tip may be easily separated from the electrode. Therefore, when a noble metal tip with an increased diameter is used, the energy of the applied laser beam must be increased to ensure sufficiently high separation resistance of the noble metal tip. However, when the energy of the applied laser beam is increased, the exposed area of a fused portion between the noble metal tip and the electrode increases, and therefore the surface area of the noble metal tip decreases; i.e., the height of the noble metal tip from a discharge surface to the end of the fused portion decreases. In this case, a consumable portion of the noble metal tip decreases, so that the effect of extending the service life of the spark plug by increasing the diameter of the noble metal tip is reduced.
An advantage of the present invention is a spark plug in which a noble metal tip is provided on at least one electrode of a ground electrode and a center electrode (the at least one electrode may hereinafter be referred to simply as an electrode) and which is excellent in durability by virtue of the noble metal tip having a sufficiently high wear resistance and a sufficiently high separation resistance.